webnn: Refactor the task environment setup defined in multiple places
to the main webnn_test_environment class.

This work is in preparation to move some WebNN objects out of GpuMain.
We need to be able to control the task environment when destruction
of WebNN objects will be done async via posted tasks, especially in tests.

Bug: 436531065
Change-Id: I8467a95b8f15790a4d7f49858167385e6070afe9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6980611
Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>
Cr-Commit-Position: refs/heads/main@{#1522575}